Taken from the
3rd Volume of Joseph Priestley's"Experiments and Observations on
Different Kinds of Air"(This edition of Priestley's
work was published in 3 volumes in 1790. I havetried to copy the original, page
for page, as faithfully as possible.)

B O O K IX

EXPERIMENTS AND OBSERVATIONS RELATING TO VEGETATION
AND RESPIRATION.

PART I

OBSERVATIONS AND EXPERIMENTS RELATING TO VEGETATION.

SECTION I

On the Restoration of Air in which a Candle hasburned out, by Vegetation.

IT is well known that flame cannot subsist long without
change of air, so that the common air is necessary to it, except in the
case of substances, into the composition of which nitre enters; for these
will burn in vacuo, in fixed air, and even

p. 248

under water, as is evident in some rockets, which are
made for this purpose. It is generally said, that an ordinary candle consumes,
as it is called, about a gallon in a minute. Considering this amazing consumption
of air, by fires of all kinds, volcanoes &c. it becomes a great object
of philosophical inquiry, to ascertain what change is made in the constitution
of the air by flame, and to discover what provision there is in nature
for remedying the injury which the atmosphere receives by this means. Having read, in the Memoirs of the
Philosophical Society at Turin, vol. I. p. 41, that air in which candles
had burned out was perfectly restored, so that other candles would burn
in it again as well as ever, after having been exposed to a considerable
degree of cold, and likewise after having been compressed in bladders
(for the cold had been supposed to have produced this effect by nothing
but condensation) I repeated those experiments, and did, indeed,
find, that when I compressed the air in bladders, as the Count de
Saluce, who made the observation, had done, the experiment succeeded: but
having had sufficient reason to distrust bladders, I compres- sed the air
in a glass vessel standing in water; and then I found, that this process
is altogether ineffectual for the purpose. I kept the air compressed much
more, and much longer, than

p. 249

the Count had done, but without producing any alteration
in it. I also find, that a greater degree of cold than that which he applied,
and of longer continuance, did by no means restore this kind of air: for
when I had exposed the phials which contained it a whole night, in which
the frost was very intense; and also when I kept it surrounded with a mixture
of snow and salt, I found it, in all respects, the same as before. It is also advanced, in the same Memoir,
p. 41, that heat only, as the reverse of cold, renders air
unfit for candles burning in it. But I repeated the experiment of the Count
for that purpose, without finding any such effect from it. I also remember
that, many years ago, I filled an exhausted receiver with air, which had
passed through a glass tube made red hot, and found that a candle would
burn in it perfectly well. Also, rarefaction by the air pump does not injure
air in the least degree. Though this experiment failed, I have
been so happy, as by accident to have hit upon a method of restoring air,
which has been injured by the burning of candles, and to have discovered
at least one of the restoratives which nature employs for this purpose.
It is vegetation. This restoration of vitiated air, I conjecture,
is effected by plants imbibing the phlogistic matter with which it is

p. 250

overloaded by the burning of inflammable bodies. But
whether there be any foundation for this conjecture or not, the fact is,
I think, indisputable. I shallintroduce the account
of my experiments on this subject, by reciting some of the observations
which I made on the growing of plants in confined air, which led to this
discovery. One might have imagined that, since
common air is necessary to vegetable, as well as to animal life, both plants
and animals had affected it in the same manner; and I own I had that expectation,
when I first put a sprig of mint into a glass jar, standing inverted in
a vessel of water; but when it had continued growing there for some months,
I found that the air would neither extinguish a candle, nor was it at all
inconvenient to a mouse, which I put into it. The plant was not affected any otherwise
than was the necessary consequence of its confined situation; for plants
growing in several other kinds of air, were all affected in the very same
manner. Every succession of leaves was more diminished in size than the
preceding, till, at length, they came to be no bigger than the heads of
pretty small pins. The root decayed, and the stalk also, beginning from
the root; and yet the plant continued to grow upwards, drawing its nourishment
through a black and rotten stem. In the third or fourth set of leaves,

p. 251

long and white hairy filaments grew from the insertion
of each hair, and sometimes from the body of the stem, shooting out as
far as the vessel in which it grew would permit, which, in my experiments,
was about two inches. In this manner a sprig of mint lived, the old plant
decaying, and new ones shooting up in its place, but less and less continually,
all the summer season. In repeating this experiment, care
must be taken to draw away all the dead leaves from about the plant, left
they should putrefy, and affect the air. I have found that a fresh cabbage
leaf, put under a glass vessel filled with common air, for the space of
one night only, has so affected the air, that a candle would not burn in
it the next morning, and yet the leaf had not acquired any smell of putrefaction. Finding, that candles would burn very
well in air in which plants had grown a long time, and having had some
reason to think, that there was something attending vegetation, which restored
air that had been injured by respiration, I thought it was possible that
the same process might also restore the air that had been injured by the
burning of candles.Accordingly, on the 17th of August 1771, I Put a sprig
of mint into a quantity of air, in which a wax candle had burned out, and
found that, on the27th of the same month, another
candle burned

p. 252

perfectly well in it. This experiment I repeated, without
the least variation in the event, not less than eight or ten times in the
remainder of the summer. Several times I divided the quantity
of air in which the candle had burned out, into two parts, and putting
the plant into one of them, left the other in the same exposure, contained,
also, in a glass vessel immersed in water, but without any plant; and never
failed to find, that a candle would burn in the former, but not in the
latter. I generally found that five or six
days were sufficient to restore this air, when the plant was in its vigour;
whereas I have kept this kind of air in glass vessels, immersed in water
many months, without being able to perceive that the least alteration had
been made in it. I have also tried a great variety of experiments upon
it, as by condensing, rarefying, exposing to the light and heat, &c.
and throwing into it the effluvia of many different substances, but without
any effect. Experiments made in the year I772,
abundantly confirmed my conclusion concerning the restoration of air, in
which candles had burned out by plants growing in it. The first of these
experiments was made in the month of May; and they were frequently repeated
in that and the two following months, without a single failure.

p. 253

For this purpose I used the flames
of different substances, though I generally used wax or tallow candles.
On the 24th of June the experiment succeeded perfectly well with air in
which spirit of wine had burned out, and on the 27th of the same month
it succeeded equally well with air in which brimstone matches had burned
out, an effect of which I had despaired the preceding year. This restoration of air, I found,
depended upon the vegetating state of the plant; for though I kept
a great number of the fresh leaves of mint in a small quantity of air in
which candles had burned out, and changed them frequently, for a long space
of time, I could perceive no melioration in the state of the air. This remarkable effect does not depend
upon any thing peculiar to mint, which was the plant that I always
made use of till July 1772; for on the 16th of that month, I found a quantity
of this kind of air to be perfectly restored by sprigs of balm which
had grown in it from the 7th of the same month. That this restoration of air was not
owing to any aromatic effluvia of these two plants, not only appeared
by the essential oil of mint having no sensible effect of this kind;
but from the equally complete restoration of this vitiated air by the plant
called groundsel, which is usually ranked among the weeds, and

p. 254

has an offensive smell. This was the result of an experiment
made the 16th of July, when the plant had been growing in the burned air
from the 8th of the same month. Besides, the plant which I have found to
be the most effectual of any that I have tried for this purpose is spinach,
which is of quick growth, but will Seldom thrive long in water. One jar
of burned air was perfectly restored by this plant in four days, and another
in two days. This last was observed on the 22d of July. In general, this effect may be presumed
to have taken place in much less time than I have mentioned; because I
never chose to make a trial of the air, till I was pretty sure, from preceding
observations, that the event which I had expected must have taken place,
if it would succeed at all; lest, returning back that part of the air on
which I made the trial, and which would thereby necessarily receive a small
mixture of common air, the experiment might not be judged to be quite fair;
though I myself might be sufficiently satisfied with respect to the allowance
that was to be made for that small imperfection.

p. 255

S E C T I O N II.

Of the Restoration of Air infested with animal Respiration, or Putrefaction, by Vegetation.

THAT candles will burn only a certain time in a given
quantity of air is a fact not better known, than it is that animals can
live only a certain time in it; but the cause of the death of the animal
is not better known than that of the extinction of flame in the same circumstances;
and when once any quantity of air has been rendered noxious by animals
breathing in it as long as they could, I do not know that any methods have
been discovered of rendering it fit for breathing again. It is evident,
however, that there must be some provision in nature for this purpose,
as well as for that of rendering the air fit for sustaining flame; for
without it the whole mass of the atmosphere would, in time, become unfit
for the purpose of animal life; and yet there is no reason to think that
it is, at prevent, at all less fit for respiration than it has ever been.
I flatter myself, however, that I have hit upon one of the methods employed
by nature for this great purpose. How many others there may be, I cannot
tell.